Dual-use sensor for rate responsive pacing and heart sound monitoring

ABSTRACT

An implantable medical device includes a dual-use sensor such as a single accelerometer that senses an acceleration signal. A sensor processing circuit processes the acceleration signal to produce an activity level signal and a heart sound signal. The implantable medical device provides for rate responsive pacing in which at least one pacing parameter, such as the pacing interval, is dynamically adjusted based on the physical activity level. The implantable medical device also uses the heart sounds for pacing control purposes or transmits a heart sound signal to an external system for pacing control and/or diagnostic purposes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/703,175, filed on Nov. 6, 2003, the specification of which isincorporated herein by reference.

This application is related to co-pending, commonly assigned U.S. patentapplication Ser. No. 10/307,896, entitled “PHONOCARDIOGRAPHICIMAGE-BASED ATRIOVENTRICULAR DELAY OPTIMIZATION,” filed on Dec. 2, 2002,and U.S. patent application Ser. No. 10/334,694, entitled “METHOD ANDAPPARATUS FOR MONITORING OF DIASTOLIC HEMODYNAMICS,” filed on Dec. 30,2002, which are hereby incorporated by reference.

TECHNICAL FIELD

This document relates generally to cardiac rhythm management systems andparticularly, but not by way of limitation, to such a system sensingheart sounds and delivering rate responsive pacing.

BACKGROUND

A heart is the center of a person's circulatory system. It includes anelectro-mechanical system performing two major pumping functions. Theleft portions of the heart draw oxygenated blood from the lungs and pumpit to the organs of the body to provide the organs with their metabolicneeds for oxygen. The right portions of the heart draw deoxygenatedblood from the organs and pump it into the lungs where the blood getsoxygenated. The body's metabolic need for oxygen increases with thebody's physical activity level. The pumping functions are accomplishedby contractions of the myocardium (heart muscles). An increase in thebody's metabolic need for oxygen is satisfied primarily by a higherfrequency of the contractions, i.e., a higher heart rate. In a normalheart, the sinoatrial node, the heart's natural pacemaker, generateselectrical impulses, called action potentials, that propagate through anelectrical conduction system to various regions of the heart to excitemyocardial tissues in these regions. Coordinated delays in thepropagations of the action potentials in a normal electrical conductionsystem cause the various regions of the heart to contract in synchronysuch that the pumping functions are performed efficiently.

The functions of the sinoatrial node and the electrical conductionsystem are indicated by electrocardiography (ECG) with at least twoelectrodes placed in or about the heart to sense the action potentials.When the heart contracts irregularly or otherwise abnormally, one ormore ECG signals indicate that contractions at various cardiac regionsare chaotic and unsynchronized. Such conditions are known as cardiacarrhythmias. Cardiac arrhythmias result in a reduced pumping efficiencyof the heart, and hence, diminished blood circulation.

Pacing therapy treats cardiac arrhythmias by using an implantablepacemaker to deliver electrical pulses that substitute for the actionpotentials to excite the myocardium, thereby restoring the functions ofthe sinoatrial note and/or the natural electrical conduction system. Toensure that the body receives sufficient oxygen to satisfy its metabolicneeds, a pacing mode referred to as rate responsive pacing, or rateadaptive pacing, uses an indication of the body's physical activitylevel to dynamically adjust the pacing rate, which determines thefrequency of the contractions.

Various mechanical functions of the heart, as well as electromechanicalassociation between the electrical conduction system and the myocardium,are indicated by heart sounds. For example, amplitudes of the thirdheart sound (S3) and fourth heart sound (S4) are related to filingpressures of the left ventricle during diastole. Fundamental frequenciesof S3 and S4 are related to ventricular stiffness and dimension. Chronicchanges in S3 amplitude is correlated to left ventricular chamberstiffness and degree of restrictive filling. Change in the intervalbetween atrial contraction and S4 is correlated to the changes in leftventricular end of diastolic pressure. Such parameters, being correlatedto the heart's mechanical properties and electromechanical association,quantitatively indicate abnormal cardiac conditions such as heartfailure, including degrees of severity, and need of appropriatetherapies.

For these and other reasons, there is a need for an implantablepacemaker that senses the body's physical activity level and the heartsounds. Implantability requires that any circuit or functional module ofthe implantable pacemaker to be designed for the minimum size and energyconsumption.

SUMMARY

An implantable medical device includes a dual-use sensor such as asingle accelerometer that senses an acceleration signal. A sensorprocessing circuit processes the acceleration signal to produce anactivity level signal and a heart sound signal. The implantable medicaldevice provides for rate responsive pacing in which at least one pacingparameter, such as the pacing interval, is dynamically adjusted based onthe physical activity level. The implantable medical device also usesthe heart sounds for pacing control purposes or transmits a heart soundsignal to an external system for pacing control and/or diagnosticpurposes.

In one embodiment, a cardiac rhythm management system includes a sensingcircuit, a pacing circuit, a dual-use sensor, a sensor processingcircuit, and a controller. The sensing circuit senses at least oneelectrogram. The pacing circuit delivers pacing pulses. The dual-usesensor senses a signal indicative of activities and heart sounds. Thesensor processing circuit produces an activity level signal and a heartsound signal from the sensed signal. The controller includes a rateresponsive pacing algorithm execution module dynamically adjusting atleast a pacing interval based on at least the activity level signal.

In one embodiment, a cardiac rhythm management system includes anaccelerometer, a processing circuit, and a controller. The accelerometersenses an acceleration signal indicative of physical activities andheart sounds. The processing circuit has an input to receive theacceleration signal, an amplifier, and a band-pass filter. The amplifierhas a programmable gain. The band-pass filter has one or more cutofffrequencies programmable for producing an activity level signal duringfirst time periods and producing a heart sound signal during second timeperiods. The controller includes a processing circuit programming moduleadapted to program the gain and the cutoff frequencies.

In one embodiment, a cardiac rhythm management system includes anaccelerometer to sense an acceleration signal, a first processingcircuit, and a second processing circuit. The first processing circuitincludes a first input to receive the acceleration signal, a firstoutput indicative of a physical activity level, and a firstgain-and-filter circuit to provide for a first gain and a first set ofcutoff frequencies. The second processing circuit includes a secondinput to receive the acceleration signal, a second output indicative ofheart sounds, and a second gain-and-filter circuit to provide for asecond gain and a second set of cutoff frequencies.

In one embodiment, a signal indicative of activities and heart sounds issensed using a single implantable sensor. The sensed signal is processedto produce an activity level signal and a heart sound signal. A rateresponsive pacing algorithm dynamically adjusts at least one pacingparameter based on the activity level signal. At least one type of heartsounds is detected from the heart sound signal.

In one embodiment, an acceleration signal indicative of an activitylevel and heart sounds is sensed. An amplifier is programmed with afirst gain suitable for sensing the activity level for a first timeperiod. A band-pass filter is programmed with a first set of cutofffrequencies suitable for sensing the activity level for the first timeperiod. The sensed acceleration signal is amplified and filtered toproduce an activity level signal. The amplifier is programmed with asecond gain suitable for sensing the heart sounds for a second timeperiod. The band-pass filter is programmed with a second set of cutofffrequencies suitable for sensing the heart sounds for the second timeperiod. The sensed acceleration signal is amplified and filtered toproduce a heart sound signal.

In one embodiment, an acceleration signal is sensed. An activity levelsignal and a heart sound signal is produced concurrently from theacceleration signal by amplifying and filtering. The acceleration signalis amplified with a first gain and filtered with a first set of cutofffrequencies suitable for producing the activity level signal, and isamplified with a second gain and filtered with a second set of cutofffrequencies suitable for producing the heart sound signal.

This Summary is an overview of some of the teachings of the presentapplication and not intended to be an exclusive or exhaustive treatmentof the present subject matter. Further details about the present subjectmatter are found in the detailed description and appended claims. Otheraspects of the invention will be apparent to persons skilled in the artupon reading and understanding the following detailed description andviewing the drawings that form a part thereof, each of which are not tobe taken in a limiting sense. The scope of the present invention isdefined by the appended claims and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe similar components throughout the several views. The drawingsillustrate generally, by way of example, but not by way of limitation,various embodiments discussed in the present document.

FIG. 1 is a block diagram illustrating an embodiment of a cardiac rhythmmanagement system, including an implantable medical device, and portionsof an environment in which it is used.

FIG. 2A is a block diagram illustrating an embodiment of a circuit ofthe implantable medical device.

FIG. 2B is a block diagram illustrating an embodiment of a controllerbeing a part of the circuit of the implantable medical device.

FIG. 3 is a block diagram illustrating an embodiment of a circuitincluding a dual-use sensor and a sensor processing circuit for sensinga physical activity level and a heart sound signal.

FIG. 4 is a block diagram illustrating another embodiment of the circuitincluding the dual-use sensor and the sensor processing circuit forsensing the physical activity level and the heart sound signal.

FIG. 5A is a block diagram illustrating an embodiment of the circuit ofFIG. 3 in which the sensor processing circuit includes an additionalpreconditioning circuit.

FIG. 5B is a block diagram illustrating an embodiment of the circuit ofFIG. 4 in which the sensor processing circuit includes an additionalpreconditioning circuit.

FIG. 5C is a block diagram illustrating an embodiment of a circuit ofthe additional preconditioning circuit.

FIG. 6 is a flow chart illustrating an embodiment of a method forsensing the physical activity level and the heart sounds by using thecircuit of FIGS. 3 and 5.

FIG. 7 is a flow chart illustrating an embodiment of a method forsensing the physical activity level and the heart sounds by using thecircuit of FIGS. 4 and 5.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are discussed in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description provides examples,and the scope of the present invention is defined by the appended claimsand their equivalents.

It should be noted that references to “an”, “one”, or “various”embodiments in this disclosure are not necessarily to the sameembodiment, and such references contemplate more than one embodiment.

This document discusses, among other things, a cardiac rhythm managementsystem including a dual-use sensor, such as a single accelerometer, forrate responsive pacing and heart sound sensing. The cardiac rhythmmanagement system includes, for example, an implantable medical deviceincluding such as a pacemaker, a pacemaker/defibrillator, apacemaker/drug delivery device, or a cardiac resynchronization therapy(CRT) device. The implantable medical device provides for rateresponsive pacing and heart sound sensing.

Rate response pacing, also referred to as rate adaptive pacing, uses anindication of a patient's gross physical activity level to adjust apacing rate, such that the cardiac output as a result of pacing meets orapproaches the patient's metabolic need. One example of rate responsivepacing using acceleration to adjust the pacing rate is discussed in U.S.Pat. No. 5,179,947, entitled “ACCELERATION-SENSITIVE CARDIAC PACEMAKERAND METHOD OF OPERATION,” assigned to Cardiac Pacemakers, Inc., which ishereby incorporated by reference in its entirety.

Known and studied heart sounds include the “first heart sound,” or S1,the “second heart sound,” or S2, the “third heart sound,” or S3, the“fourth heart sound,” or S4, and their various sub-components. S1 isknown to be indicative of, among other things, mitral valve closure,tricuspid valve closure, and aortic valve opening. S2 is known to beindicative of, among other things, aortic valve closure and pulmonaryvalve closure. S3 is known to be a ventricular diastolic filling soundoften indicative of certain pathological conditions including heartfailure. S4 is known to be a ventricular diastolic filling soundresulted from atrial contraction and is usually indicative ofpathological conditions. The term “heart sound” hereinafter refers toany heart sound (e.g., S1) and any components thereof (e.g., M1component of S1, indicative of Mitral valve closure and Mitralregurgitation). Heart sounds are used, for example, to calculate pacingparameters for improving the patient's hemodynamic performance anddiagnosing a pathological condition such as heart failure. Examples ofsuch uses are discussed in co-pending U.S. patent application Ser. No.10/307,896, entitled “PHONOCARDIOGRAPHIC IMAGE-BASED ATRIOVENTRICULARDELAY OPTIMIZATION,” and U.S. patent application Ser. No. 10/334,694,entitled “METHOD AND APPARATUS FOR MONITORING OF DIASTOLICHEMODYNAMICS,” both assigned to Cardiac Pacemakers, Inc., thespecifications of which are incorporated herein by reference in theirentirety.

An accelerometer can be used to sense both the physical activity levelfor rate responsive pacing and the heart sounds because the two signalsfeature substantially distinguishable spectrums. The accelerationmeasured in the direction normal to a person's chest wall is indicativeof both the physical activity level and the heart sounds. The sensorspecifications required for sensing the physical activity level and thesensor specifications required for sensing the heart sounds, such asbandwidth, sensitivity, noise floor, robustness, size, and powerconsumption are sufficiently close such that they can be satisfied by asingle accelerometer having adequate size and power consumption for usedin an implantable medical device. A sensor processing circuit processesthe signal sensed by such an accelerometer to produce an activity levelsignal indicative of the physical activity level for rate responsivepacing and a heart sound signal from which heart sounds of each type canbe detected.

Throughout this document, a “heart sound signal” includes audible andinaudible mechanical vibrations of the heart that can be sensed with asensor such as an accelerometer. A “heart sound” refers to a recognizedevent in the heart sound signal. Unless noted otherwise, S1, S2, S3, andS4 refer to the first, second, third, and fourth heart sounds,respectively, as a heart sound type, or as one or more occurrences ofthe corresponding type heart sounds, depending on the context. An“electrogram” includes an electrocardiogram (ECG) sensed with at leastan intracardiac electrode placed in the heart or an epicardial electrodeplaced on the heart. A “user” includes a physician or other caregiverwho examines and/or treats a patient using one or more of the methodsand apparatuses discussed in the present document.

FIG. 1 is a block diagram illustrating an embodiment of a cardiac rhythmmanagement system 100, and portions of an environment in which it isused. System 100 includes an implantable medical device 110, a leadsystem 108, an external system 170, and a wireless telemetry link 160.

After implantation, implantable medical device 110 operates within abody 102 to sense activities of a heart 105 and deliver one or moretherapies to heart 105. Implantable medical device 110 includes apacemaker capable of rate responsive pacing. In one embodiment,implantable medical device 110 is an implantable pacemaker. In anotherembodiment, implantable medical device 110 includes a pacemaker moduleand one or more other therapeutic modules, such as acardioversion-defibrillation module and a drug delivery module. In onespecific embodiment, the pacemaker includes a cardiac resynchronizationtherapy module capable of delivering multi-site biventricular pacing.Implantable medical device 110 senses an activity level for rateresponsive pacing purposes and heart sounds for various diagnosticand/or therapy control purposes. A dual-use sensor such as anaccelerometer is used for sensing both the activity level and the heartsounds. In one embodiment, the dual-use sensor is within implantablemedical device 110.

Lead system 108 provides one or more electrical connections betweenimplantable medical device 110 and heart 105. It includes one ore morepacing leads each having one or more electrodes for electrogram sensingand pacing pulse delivery. In one embodiment, the dual-use sensor isincorporated into a lead of lead system 108 and connected to implantablemedical device through the lead.

External system 170 communicates with implantable medical device 110. Itallows a user and/or a patient to communicate and/or to control theoperation of implantable medical device 110. In one embodiment, externalsystem 170 includes an external programmer. In another embodiment,external system 170 includes an advanced patient management system, suchas discussed in U.S. patent application Ser. No. 10/323,604, entitled“ADVANCED PATIENT MANAGEMENT FOR DEFINING, IDENTIFYING AND USINGPREDETERMINED HEALTH-RELATED EVENTS,” assigned to Cardiac Pacemakers,Inc., the specification of which is incorporated herein by reference inits entirety. The advanced patient management system allows the user toaccess implantable medical device 110 from a remote location.

Telemetry link 160 provides for data transmissions between implantablemedical device 110 and external system 170. In one embodiment, telemetrylink 160 is an inductive telemetry link. In an alternative embodiment,telemetry link 160 is a far-field radio-frequency telemetry link.Telemetry link 160 provides for data transmission from implantablemedical device 110 to external system 170. This may include, forexample, transmitting real-time physiological data acquired byimplantable medical device 110, extracting physiological data acquiredby and stored in implantable medical device 110, extracting therapyhistory data stored in implantable medical device 110, and extractingdata indicating an operational status of implantable medical device 110(e.g., battery status and lead impedance). Telemetry link 160 alsoprovides for data transmission from external system 170 to implantablemedical device 110. This may include, for example, programmingimplantable medical device 110 to acquire physiological data,programming implantable medical device 110 to perform at least oneself-diagnostic test (such as for a battery status and lead impedancestatus), and programming implantable medical device 110 to deliver atleast one therapy. Examples of signals represented by the physiologicaldata include, but are not limited to, electrograms, heart sounds orsignals indicative of heart sounds, activity level signals, andrespiratory signals. In one embodiment, the physiological data alsoinclude parameters measured from one or more of these signals. In oneembodiment, external system 170 or a user determines and/or adjusts atherapy based on these signals and/or physiological data.

FIG. 2A is a block diagram illustrating an embodiment of a circuit ofimplantable medical device 110. Implantable medical device 110 includesa sensing circuit 212, a pacing circuit 214, a dual-use sensor 216, asensor processing circuit 218, a heart sound detector 220, an implantcontroller 222, a memory circuit 224, and an implant telemetry module226. In one embodiment, these circuit elements, and possibly additionalcircuit elements of implantable medical device 110, are encapsulated ina hermetically sealed implantable housing. In another embodiment, someof these circuit elements, such as dual-use sensor 216 or implanttelemetry module 224, are located outside of the hermetically sealedimplantable housing.

Sensing circuit 212 and pacing circuit 214 are both electrically coupledto heart 105 via lead system 108. Sensing circuit 212 includes anamplifier circuit suitable for sensing one or more electrograms fromheart 108 through lead system 108. Pacing circuit 214 includes a pulsegenerator generating electrical pacing pulses that are delivered toheart 105 through lead system 108.

Dual-use sensor 216 senses a signal indicative of two activities,events, or quantities having distinguishable frequency characteristics.The term “dual-use” refers to the fact that the two activities, events,or quantities are extracted separately from the signal and used fordifferent purposes serving the functions of system 100. In oneembodiment, dual-use sensor 216 includes a single accelerometer thatsenses an acceleration signal indicative of a patient's gross physicalactivity level and heart sounds of the patient. In one embodiment,dual-use sensor 216 is an integrated circuit accelerometer. A specificexample of such an integrated circuit accelerometer is a piezoelectricaccelerometer made by Endevco Corporation (Model 12 PicochipAccelerometer). Other examples include piezoresistive and capacitiveaccelerometers. In one embodiment, dual-use sensor 216 is encapsulatedin the hermetically sealed implantable housing. This embodiment providesthe accelerometer with an environment ensuring a stable operation. In analternative embodiment, dual-use sensor 216 is incorporated into a leadof lead system 108. This embodiment allows the accelerometer to belocated in or near heart 105, thus being more sensitive to the heart'smechanical activities such as vibrations (heart sounds).

Sensor processing circuit 218 processes the acceleration signal toproduce an activity level signal indicative of the patient's grossphysical activity level and a heart sound signal indicative of thepatient's heart sounds. Embodiments of sensor processing circuit 218 arediscussed below, with reference to FIGS. 3-5.

In one embodiment, heart sound detector 220 detects heart sounds fromthe heart sound signal produced by sensor processing circuit 218. In oneembodiment, implantable controller 222 receives the detected heartsounds use it, in addition to the activity level signal, for rateresponsive pacing purposes. In a further or alternative embodiment,implantable controller 222 uses the detected heart sounds for adjustingatrioventricular and/or interventricular pacing delays, such as in acardiac resynchronization therapy for heart failure. The detected heartsounds, and/or parameters measured from the detected heart sounds, arealso transmitted to external system 170 through telemetry link 160 forfurther analysis by the system or the user. In an alternativeembodiment, the heart sound signal is transmitted to external system 170though telemetry link 160. External system 170 detects and analyzes theheart sound signal for diagnostic and/or pacing control purposes. Heartsound detector 220 is configured and/or programmed by external system170 to detect one or more of the S1, S2, S3, and S4 type heart sounds.

Implant controller 222 controls the operation of the entire implantablemedical device 110. An embodiment of implantable device 222 is discussedbelow, with reference to FIG. 2B. In one embodiment, implant controller222 is implemented using a microprocessor. Memory circuit 224 provides astorage medium for a device control code, parameters for the operationof implantable medical device 110, and the data acquired by implantablemedical device 110. In one embodiment, memory circuit 224 includes abuffer for storing the signal sensed by dual-use sensor 216, theactivity level signal produced by sensor processing circuit 218, and/orthe heart sound signal produced by sensor processing circuit 218. Inanother embodiment, the buffer also stores the one or more electrogramssensed by sensing circuit 212. In a further embodiment, implantcontroller 222 includes an event detector to detect cardiac events and amaker generator to generate event markers representing the cardiacevents. Examples of such cardiac events include sensed events (intrinsicdepolarizations) and paced events (paced contractions or pulsedeliveries) associated with one or more cardiac sites. Each event markis indicative of the type and the timing of one cardiac event. In thisembodiment, the buffer further stores the event markers. In oneembodiment, the activity level signal and/or the heart sound signal aresynchronized with the event markers such that the event markers serve asa timing reference relating the activity level and/or heart sound to thecardiac events. In one embodiment, implant controller 222 includes ananalog-to-digital converter to digitize one or more of the signal sensedby dual-use sensor 216, the activity level signal, the heart soundsignal, and the electrograms for storage and/or further processing. Theanalog-to-digital converter has a programmable sampling rate.Implantable controller 222 includes a digitization control module tocontrol this sampling rate. In one embodiment, the sampling rate isprogrammable through external system 170.

Implant telemetry module 226 includes an antenna and a transceiver tosupport two-way communications with external system 170 via telemetrylink 160. In one embodiment, one or more of the electrograms, the eventmarkers, the signal sensed by dual-use sensor 216, the activity levelsignal, and the heart sound signal are transmitted to external system170 in real time. In another embodiment, one or more of theelectrograms, the event markers, the signal sensed by dual-use sensor216, the activity level signal, and the heart sound signal are stored inthe buffer of memory circuit 224 and retrieved from the buffer whenneeded. In one embodiment, the retrieval occurs at predetermined timesas controlled by implant controller 222. In another embodiment, theretrieval occurs in response to a command from external system 170.

FIG. 2B is a block diagram illustrating an embodiment of implantcontroller 222. Implant controller 222 executes the device control codestored in memory circuit 224. It includes, among other control modules,a rate responsive pacing algorithm execution module 230 and a sensorprocessing circuit programming module 232.

Rate responsive pacing algorithm execution module 230 controls thetiming of the pacing pulse delivery from pacing circuit 214, based onpredefined pacing logic and timing rules and one or more of the activitylevel signal, the sensed electrograms, timing of previous pacing pulsedeliveries, the detected heart sounds, and possibly other physiologicalsignals indicative of electrical events, mechanical activities, and/orhemodynamic performance of heart 105. It includes a pacing intervalcalculator to calculate a pacing interval based on at least the activitylevel signal and predetermined maximum and minimum pacing intervals.When the pacing interval calculator produces a new value for the pacinginterval, rate responsive pacing algorithm execution module 230 updatesthe pacing interval with the new value. In one embodiment, rateresponsive pacing algorithm execution module 230 performs thecalculation and the update dynamically, for each and every heart beat.

In one embodiment, sensor processing circuit programming module 232controls the timing, gain, and/or frequency responses of sensorprocessing circuit 218 to produce the activity level signal and theheart sound signal. The programming of sensor processing circuit 218 isdiscussed below with reference to FIGS. 3 and 4.

FIG. 3 is a block diagram illustrating an embodiment of a circuitincluding an accelerometer 316 for sensing the acceleration signal and asensor processing circuit 318 for producing the activity level signaland the heart sound signal from the acceleration signal. Accelerometer316 is one embodiment of dual-use sensor 216 or a portion thereof Sensorprocessing circuit 318 is one embodiment of sensor processing circuit218 or a portion thereof.

Sensor processing circuit 318 includes an amplifier 340, a band-passfilter 342, and a demultiplexer (DEMUX) 343. It produces the activitylevel signal and the heart sound signal from the acceleration signalsense by accelerometer 316 on a time-sharing basis. During predeterminedfirst time periods, sensor processing circuit 318 produces the activitylevel signal. During predetermined second time periods, sensorprocessing circuit 318 produces the heart sound signal. The first andsecond time periods do not overlap. Sensor processing circuitprogramming module 232 controls the first time periods for producing theactivity level signal and the second time periods for producing theheart sound signal by programming the gain of amplifier 340, the cutofffrequencies of band-pass filter 342, and the connections withindemultiplexer 343. Thus, sensor processing circuit 318 has an input toreceive the acceleration signal, an output representative of theactivity level signal during the first periods, and another outputrepresentative of the heart sound signal during the second periods. Inone embodiment, the gain and/or the cutoff frequencies are predeterminedand stored in memory circuit 224. In one specific embodiment, the gainand/or the cutoff frequencies are empirically determined based on datacollected from the patient treated with system 100, and programmed intoimplantable medical device 110 by using external system 170. In oneembodiment, the gain and/or the cutoff frequencies are adjustable by theuser, when necessary, after the implantation of implantable medicaldevice 110. The adjustments may become necessary when, for example, therange of the amplitude of the sensed acceleration signal hassignificantly changed, or when a different type of the heart sound issought. Demultiplexer 343 receives the output of band-pass filter 342and provides two outputs separately representing the activity levelsignal and the heart sound signal.

For producing the activity level signal, the gain of amplifier 340 is afirst gain programmable in the range of 100 to 500. The cutofffrequencies of band-pass filter 342 are a first set of cutofffrequencies including a first low cutoff frequency programmable in arange of 0.5 Hz to 2 Hz and a first high cutoff frequency programmablein a range of 5 Hz to 15 Hz. In one specific embodiment, sensorprocessing circuit programming module 232 programs the first gain to125, the first low cutoff frequency to 1 Hz, and the first high cutofffrequency to 10 Hz during the predetermined first time periods. Forproducing the heart sound signal, the gain of amplifier 340 is a secondgain programmable in the range of 500 to 2000. The cutoff frequencies ofband-pass filter 342 are a second set of cutoff frequencies including asecond low cutoff frequency programmable in a range of 5 Hz to 10 Hz anda second high cutoff frequency programmable in a range of 50 Hz to 200Hz. In one specific embodiment, sensor processing circuit programmingmodule 232 programs the second gain to 1000, the second low cutofffrequency to 10 Hz, and the second high cutoff frequency to 100 Hzduring the predetermined second time periods.

Sensor processing circuit 318 requires only one set of an amplifier anda filter to produce both the activity level signal and the heart soundsignal. It is suitable for applications in which the activity level andthe heart sounds need not be sensed concurrently. For an implantablepacemaker that already requires an accelerometer for the purpose of rateresponsive pacing, sensor processing circuit 318 provides for heartsound sensing with minimal additional requirement for circuit size andenergy consumption.

FIG. 4 is a block diagram illustrating another embodiment of the circuitincluding accelerometer 316 for sensing the acceleration signal and asensor processing circuit 418 for concurrently producing the activitylevel signal and the heart sound signal from the acceleration signal.Accelerometer 316 is one embodiment of dual-use sensor 216 or a portionthereof. Sensor processing circuit 418 is one embodiment of sensorprocessing circuit 218 or a portion thereof.

Sensor processing circuit 418 includes a first processing circuit 450Afor producing the activity level signal and a second processing circuit450B for producing the heart sound signal. First processing circuit 450Aincludes a first amplifier 440A having a first gain and a firstband-pass filter 442A having a first set of cutoff frequencies. Secondprocessing circuit 450B includes a second amplifier 440B having a secondgain and a second band-pass filter 442B having a second set of cutofffrequencies. First processing circuit 450A and second processing circuit450B operate in parallel to allow concurrent sensing of the physicalactivity level and the heart sounds. Thus, sensor processing circuit 418has an input to receive the acceleration signal, an outputrepresentative of the activity level signal, and another outputrepresentative of the heart sound signal. In one embodiment, the gainsand/or the cutoffs frequencies are predetermined and stored in memorycircuit 224. While there is no need to program the gains and the cutofffrequencies for the time-sharing purpose, in one embodiment, the gainsand/or the cutoffs frequencies are programmable to ensure proper sensingunder each patient's particular circumstances. In one embodiment, thegain and/or the cutoff frequencies are empirically determined based ondata collected from the patient, and programmed into implantable medicaldevice 110 by using external system 170. In one embodiment, the gainand/or the cutoff frequencies are adjustable by the user, whennecessary, after the implantation of implantable medical device 110.

The gain of amplifier 440A (the first gain) is programmable in the rangeof 100 to 500. The cutoff frequencies of band-pass filter 442A (thefirst set of cutoff frequencies) include a first low cutoff frequencyprogrammable in a range of 0.5 Hz to 2 Hz and a first high cutofffrequency programmable in a range of 5 Hz to 15 Hz. In one specificembodiment, sensor processing circuit programming module 232 programsthe first gain to 125, the first low cutoff frequency to 1 Hz, and thefirst high cutoff frequency to 10 Hz during the predetermined first timeperiods. The gain of amplifier 440B (the second gain) is programmable inthe range of 500 to 2000. The cutoff frequencies of band-pass filter442B (the second set of cutoff frequencies) include a second low cutofffrequency programmable in a range of 5 Hz to 10 Hz and a second highcutoff frequency programmable in a range of 50 Hz to 200 Hz. In onespecific embodiment, sensor processing circuit programming module 232programs the second gain to 1000, the second low cutoff frequency to 10Hz, and the second high cutoff frequency to 100 Hz during thepredetermined second time periods.

Sensor processing circuit 418 allows concurrent sensing of the activitylevel and the heart sounds. With sensor processing circuit 418, dual-usesensor 216 is usable for sensing the activity level and the heart soundssimultaneously when needed.

FIG. 5A is a block diagram illustrating an embodiment of the circuit ofFIG. 3 with an additional preconditioning circuit 552. In thisembodiment, sensor processing circuit 218 includes preconditioningcircuit 552 with its input connected to accelerometer 316 and its outputconnected to sensor processing circuit 318.

FIG. 5B is a block diagram illustrating an embodiment of the circuit ofFIG. 4 with an additional preconditioning circuit 552. In thisembodiment, sensor processing circuit 218 includes preconditioningcircuit 552 with its input connected to accelerometer 316 and its outputconnected to sensor processing circuit 418.

FIG. 5C is a block diagram illustrating an embodiment of a circuit ofpreconditioning circuit 552. Preconditioning circuit 552 provides forinitial conditioning or processing of the acceleration signal beforebeing processed for producing the activity level signal and the heartsound signal.

In one embodiment, preconditioning circuit 552 includes apreconditioning amplifier 554 having a preconditioning gain and apreconditioning band-pass filter 556 having a set of preconditioningcutoff frequencies. In one embodiment, the preconditioning gain isprogrammable in the range of 100 to 500. The overall gains for producingthe activity signal and the heart sound signal are products of thepreconditioning gain (gain of preconditioning amplifier 554) multipliedby the gains of sensor processing circuit 318 or 418 as discussed above.The gains to be programmed to sensor processing circuit 318 or 418 arecalculated by dividing the gains discussed above by the programmedpreconditioning gain. That is, the gain of amplifier 340 includes afirst gain in the range of 100 to 500 divided by the preconditioninggain for producing the activity signal, and a second gain in the rangeof 500 to 2000 divided by the preconditioning gain for producing theheart sound signal. The gain of amplifier 440A (the first gain) is inthe range of 100 to 500 divided by the preconditioning gain. The gain ofamplifier 440B (the second gain) is in the range of 500 to 2000 dividedby the preconditioning gain. In one embodiment, the set ofpreconditioning cutoff frequencies includes a low preconditioning cutofffrequency programmable in the range of 0.5 to 2 Hz, and a highpreconditioning cutoff frequency programmable in the range of 50 to 200Hz. In one specific embodiment, with sensor processing circuit 318, thepreconditioning gain is 125, the low preconditioning cutoff frequency is1 Hz, the high preconditioning cutoff frequency programmable is 100 Hz,the first gain for amplifier 340 is 1, and the second gain for amplifier340 is 8. In an alternative specific embodiment, with sensor processingcircuit 418, the preconditioning gain is 125, the low preconditioningcutoff frequency is 1 Hz, the high preconditioning cutoff frequencyprogrammable is 100 Hz, the gain for amplifier 440A is 1, and the gainfor amplifier 440B is 8.

In one embodiment, preconditioning circuit 552 further includes ananalog-to-digital converter (ADC) 558 to digitize the accelerationsignal. This allows sensor processing circuit 318 or sensor processingcircuit 418 to be implemented with digital technology. That is, theactivity level signal and the heart sound signal are digital signalsproduced from the digitized acceleration signal using digital signalprocessing. In one embodiment, ADC 558 has a programmable sampling rate,and implantable controller 222 includes a digitization control module tocontrol this sampling rate. In one further embodiment, the sampling rateis programmable through external system 170. In general, sensorprocessing circuit 218 can be implemented with hardware, software, and acombination of both.

FIG. 6 is a flow chart illustrating an embodiment of a method forsensing the physical activity level and the heart sounds with thecircuit illustrated in FIGS. 3 and 5. The method illustrates by way ofexample, but not by way of limitation, a use of the circuit thatincludes accelerometer 316, preconditioning circuit 552, and sensorprocessing circuit 318.

Accelerometer 316 senses an acceleration signal at 600. Preconditioningcircuit 552 preconditions the sensed acceleration signal at 605. In oneembodiment, preconditioning circuit 552 amplifies and filters the sensedacceleration signal. In a further embodiment, preconditioning circuit552 digitizes the sensed acceleration signal. The digitization allowssubsequent processing to be performed using digital signal processingtechnology.

Sensor processing circuit 318 is programmed with the first gain and thefirst set of cutoff frequencies for first time periods at 610. Duringthe first time periods, sensor processing circuit 318 amplifies andfilters the acceleration signal to produce the activity level signal at620. Sensor processing circuit 318 is programmed with the second gainand the second set of cutoff frequencies for second time periods at 630.During the second time periods, sensor processing circuit 318 amplifiesand filters the acceleration signal to produce the heart sound signal at640. In one embodiment, the first and second time periods are programmedinto memory circuit 224 for use by sensor processing circuit programmingmodule 232, which programs the gain and the cutoff frequencies of sensorprocessing circuit 318. The first and second time periods do notoverlap.

Rate responsive pacing algorithm execution module 230 adjusts a pacingparameter such as the pacing interval based on at least the activitylevel signal at 650. In one embodiment, rate responsive pacing algorithmexecution module 230 also adjusts the pacing interval and/or one or moreother pacing parameters based on other signals such as the electrogramsand the heart sound signal.

Heart sounds are detected from the heart sound at 660. In oneembodiment, heart sound detector 220, which is a part of implantablemedical device 110, detects the heart sounds from the heart soundsignal. In one embodiment, the detected hearts sounds are used byimplant controller 222 for pacing control purposes. In anotherembodiment, the heart sound signal and/or information extracted from thedetected heart sounds are transmitted to external system 170. In analternative embodiment, the heart sound signal is transmitted toexternal system 170, which includes a heart sound detector to detect theheart sounds. The heart sound detection includes detection ofpredetermined types of heart sounds including one or more of S1, S2, S3,and S4. The heart sounds are analyzed at 670. The analysis includesmeasurement of, for example, one or more of amplitude of any type heartsound, relative amplitude between any two types of heart sounds,duration of each type heart sound, interval between any type or types ofheart sounds, interval between any type heart sound and any typeelectrical event of the heart, fundamental frequency of each type heartsound, and harmonic frequency of each type heart sound. In oneembodiment, the results of the analysis, such as parameters generatedfrom the above measurements, are used to determine a pacing parameterbased on the heart sounds at 680. One example of determining a pacingparameter based on the heart sounds is discussed in U.S. patentapplication Ser. No. 10/307,896. In one embodiment, one or moreparameters generated from the above measurements are trended at 690. Oneexample of trending heart sound related parameters is discussed in U.S.patent application Ser. No. 10/334,694.

FIG. 7 is a flow chart illustrating an embodiment of a method forsensing the physical activity level and the heart sounds with thecircuit illustrated in FIGS. 4 and 5. The method illustrates by way ofexample, but not by way of limitation, a use of the circuit thatincludes accelerometer 316, preconditioning circuit 552, and sensorprocessing circuit 418.

Accelerometer 316 senses an acceleration signal at 700. Preconditioningcircuit 552 preconditions the sensed acceleration signal at 705. In oneembodiment, preconditioning circuit 552 amplifies and filters the sensedacceleration signal. In a further embodiment, preconditioning circuit552 digitizes the sensed acceleration signal. The digitization allowssubsequent processing to be performed using digital signal processingtechnology.

First processing circuit 450A of sensor processing circuit 418 producesthe activity level signal from the acceleration signal at 710. Thisincludes amplifying the acceleration signal with the first gain at 712and filtering the acceleration signal with the first set of cutofffrequencies at 714. Rate responsive pacing algorithm execution module230 adjusts a pacing parameter such as the pacing interval based on theactivity level signal at 720. In one embodiment, rate responsive pacingalgorithm execution module 230 also adjusts the pacing interval and/orone or more other pacing parameters based on other signals such as theelectrograms and the heart sound signal.

Second processing circuit 450B of sensor processing circuit 418 producesthe heart sound signal from the acceleration signal at 730. Thisincludes amplifying the acceleration signal with the second gain at 732and filtering the acceleration signal with the second set of cutofffrequencies at 734. Heart sounds are detected from the heart soundsignal at 740 and analyzed at 750. In one embodiment, the results of theanalysis are used to determine a pacing parameter based on the heartsounds at 760. In one embodiment, the results of the analysis are usedfor trending one or more parameters measured from the heart sounds at770. In one embodiment, step 660 is identical or similar to step 740,step 670 is identical or similar to step 750, step 680 is identical orsimilar to step 760, and step 690 is identical or similar to step 770.

Because first processing circuit 450A and second processing circuit 450Bare separate circuits producing distinguished signals from a commonsignal, step 710 (and its subsequent step 720) and step 730 (and itssubsequent steps 740, 750, 760, and 770) can be performed concurrently.

It is to be understood that the above detailed description is intendedto be illustrative, and not restrictive. For example, the sensorprocessing circuit can be expanded to produce additional signals fromthe acceleration signal, such as a respiration-indicative signal, if theadditional signals each have a distinguishable spectrum. Otherembodiments will be apparent to those of skill in the art upon readingand understanding the above description. The scope of the inventionshould, therefore, be determined with reference to the appended claims,along with the full scope of equivalents to which such claims areentitled.

1. An implantable medical device, comprising: a sensing circuitconfigured to sense at least one electrogram; a pacing circuitconfigured to deliver pacing pulses; a dual-use sensor configured tosense a signal indicative of physical activities and heart sounds; asensor processing circuit configured to produce an activity level signaland a heart sound using the sensed signal, the activity level signalindicative of a level of the physical activities, the heart sound signalindicative of the heart sounds, the sensor processing circuit including:a first set of cutoff frequencies suitable for producing the activitylevel signal; a first gain suitable for producing the activity levelsignal; a second set of cutoff frequencies suitable for producing theheart sound signal; and a second gain suitable for producing the heartsound signal; an implant controller coupled to the sensing circuit, thepacing circuit, and the sensor processing circuit, the controllerconfigured to control the delivery of the pacing pulses using theactivity level signal and the heart sound signal; and an implantablehousing encapsulating at least the sensing circuit, the pacing circuit,the sensor processing circuit, and the controller.
 2. The device ofclaim 1, wherein the dual-use sensor is a single integrated circuitaccelerometer.
 3. The device of claim 1, wherein the sensor processingcircuit is configured to produce the activity level signal during afirst time period and the heart sound signal during a second timeperiod, wherein the first time period and the second time period do notoverlap.
 4. The device of claim 3, wherein the implant controllercomprises a sensor processing circuit programming module configured toprogram the sensor processing circuit for producing the activity levelsignal using the first set of cutoff frequencies and the first gainduring a first time period and producing the heart sound signal usingthe second set of cutoff frequencies and the second gain during a secondtime period.
 5. The device of claim 4, wherein the sensor processingcircuit comprises: a band-pass filter having programmable cutofffrequencies; and an amplifier having a programmable gain, wherein theprogrammable cutoff frequencies and the programmable gain are adjustableafter implantation of the implantable medical device.
 6. The device ofclaim 1, wherein the sensor processing circuit is configured to producethe activity level signal and the heart sound signal concurrently. 7.The device of claim 6, wherein the sensor processing circuit comprises:a first band-pass filter having the first set of cutoff frequencies; afirst amplifier having the first gain; a second band-pass filter havingthe second set of cutoff frequencies; and a second amplifier having thesecond gain.
 8. The device of claim 7, wherein the first band-passfilter has a first set of programmable cutoff frequencies, and thesecond band-pass filter has a second set of programmable cutofffrequencies.
 9. The device of claim 7, wherein the first amplifierhaving a first programmable gain, and the second amplifier has a secondprogrammable gain.
 10. The method for operating an implantable medicaldevice, the method comprising: sensing a signal indicative of physicalactivities and heart sounds using a single implantable sensor; producingan activity level signal by filtering the sensed signal using firstcutoff frequencies and amplifying the sensed signal using a first gain;producing a heart sound signal by filtering the sensed signal usingsecond cutoff frequencies and amplifying the sensed signal using asecond gain; and controlling delivery of cardiac pacing pulses from theimplantable medical device using the activity level signal and the heartsound signal.
 11. The method of claim 10, wherein sensing the signalcomprises sensing an acceleration signal using an implantableaccelerometer.
 12. The method of claim 10, comprising: producing theactivity level signal during a first time period; and producing theheart sound signal during a second time period, wherein the first timeperiod and the second time period do not overlap.
 13. The method ofclaim 12, comprising producing the activity level signal and the heartsound signal using a band-pass filter having programmable cutofffrequencies and an amplifier having a programmable gain, whereinproducing the activity level signal comprises: programming theprogrammable cutoff frequencies to the first cutoff frequencies for thefirst time period; and programming the programmable gain to the firstgain for the first time period, and producing the heart sound signalcomprises: programming the programmable cutoff frequencies to the secondcutoff frequencies for the second time period; and programming theprogrammable gain to the second gain for the second time period.
 14. Themethod of claim 10, comprising producing the activity level signal andthe heart sound signal concurrently.
 15. The method of claim 14,comprising: producing the activity level signal using a first band-passfilter having the first cutoff frequencies and a first amplifier havingthe first gain; and producing the activity level signal using a secondband-pass filter having the second cutoff frequencies and a secondamplifier having the second gain.
 16. An implantable medical device,comprising: means for sensing a signal indicative of physical activitiesand heart sounds using a single implantable sensor; and means forcontrolling gains and cutoff frequencies for producing an activity levelsignal and a heart sound signal using the sensed signal.
 17. The deviceof claim 16, comprising means for controlling cardiac pacing using oneor more of the activity level signal and the heart sound signal.
 18. Thedevice of claim 17, wherein the means for controlling cardiac pacingcomprise means for determining one or more pacing parameters using theone or more of the activity level signal and the heart sound signal. 19.The device of claim 18, wherein the means for controlling gains andcutoff frequencies comprises means for controlling gains and cutofffrequencies for producing the activity level signal and the heart soundsignal concurrently.
 20. The device of claim 18, wherein the means forcontrolling gains and cutoff frequencies comprises means for controllinggains and cutoff frequencies for producing the activity level signalduring a first time period and producing the heart sound signal during asecond time period, wherein the first time period and the second timeperiod do not overlap.